The present invention generally relates to radio transmitters and, more particularly, to radio transmitters used in base stations of cellular radio systems.
In cellular radio systems, transmissions from a base station can comprise signals from many different channels which are combined prior to transmission by an antenna or antennas. These signals need to be distinctly spaced from one another in frequency so that they can be separated after reception without severe intermodulation. Conventionally, this has been achieved by using combiner/filters, commonly just called combiners, which comprise multiple tuned cavity devices that allow simultaneous transmission of signals from a plurality of transmitters at different but closely spaced frequencies by way of a single antenna.
Typically, combiners include one tuned cavity for each frequency. Each of the tuned cavities is coupled to a separate transmitter and is also coupled to an antenna. Combiners, however, have always been troublesome because numerous external influences cause the tuned cavities to become detuned. For example, normal temperature changes cause variations in the critical dimensions of these tuned cavities. Detuning of the cavities results in a substantial increase in insertion losses, thereby decreasing the amount of transmitter power that reaches the antenna. These problems are particularly acute in cellular telecommunication systems. One solution for overcoming temperature-caused detuning is to manufacture the tuning cavities from Invar, an expensive metal which must be coated with copper to provide the necessary high surface conductivity required of tuned cavities used in high frequency transmission systems.
Even this expensive solution, however, fails to prevent detuning due to other environmental factors such as variations in humidity and atmospheric pressure. Retuning the resonant frequencies of these cavities can also be accomplished manually or by computer control of tuning elements in the cavities, however, these solutions are also expensive and create other problems. Moreover, the combiner is physically bulky and takes up space in the base station which could be used for other purposes.